Power companies need smooth, dependable power. Their experience is almost entirely with fossil fuel, hydroelectric and nuclear energy sources which have very predictable outputs of electricity. As the percentage of electricity generated by wind power increases so will the amount of variability that they need to account for. The necessary spinning reserve turbines, which can be fired up quickly when the wind dies, represent an inefficiency and direct cost that reduces the marginal value of wind energy. Many resources are currently being spent trying to develop storage technologies which would enable spreading the power more evenly across time. So far, only compressed air storage and pumped hydro-storage have the capacity to practically time shift wind energy. Unfortunately, these means are very inconvenient to implement and inefficient.
Wind turbines employ two basic principles to capture energy from moving air. Aerodynamic turbines use low pressure lift; impulse turbines use drag. The differentiating factor between the two is the blade tip speed. For aerodynamic turbines, the blade tip speed is a multiple of the wind speed. In contrast, an impulse turbine can never spin faster than the wind speed. An anemometer, an often used device for measuring wind speed, is an example of an impulse type device. Conventional horizontal axis wind turbines (HAWTs) are an example of aerodynamic turbines with tip speeds reaching 100 meters per second (m/s) or 400 miles per hour.
Recent engineering and technical development in modern HAWTs have resulted in driving their efficiencies to around 35%. The theoretical maximum efficiency is limited by “Betts law” to 59%. A wind turbine cannot be 100% efficient as this would imply that the air exiting the turbine would have zero velocity and so would prevent other air from flowing through the turbine.
Efficiency factors are also misleading in that they presume a certain wind speed which is usually not accurate. For instance a HAWT may have a 30% efficiency for the wind speed of 14 m/s, but will not even spin, meaning it would have zero efficiency with a 5 m/s wind. This would be an example of what appears to be logical optimization of the wind turbine specifications. The energy in wind is a cubed function of its velocity and so optimizing wind turbine efficiencies for high wind speed results in large megawatt ratings. This is also the number that is used to describe how big a wind farm is, as in it is a 200 or a 400 Megawatt (MW) wind farm.
According to the National Renewable Energy Laboratory (NREL) wind resource map of the continental United States, the best wind resources appear to be class 3 and 4 winds over the Rocky Mountain and Great Plains states. Class 3 and 4 winds represent a yearly average of 6.7 m/s and 7.25 m/s respectively at 50 m above ground. If the average wind speed is 7 m/s, wind speeds of 14 m/s are not likely to happen even a quarter of the time, once factoring in the capacity factors. Capacity factors are based on the power curve for the particular wind turbine and wind speed data from the proposed site that the turbine will be placed on and are typically claimed to be 25 to 30%. The current paradigm of HAWTs are designed to have their highest efficiencies in the higher wind speed ranges, which makes sense in the context of the velocity cubed section of the wind power equation. The goal is to be most efficient when there is the most energy to harvest. This results in high MW ratings for the turbines but results in low capacity factors, meaning that the turbine will generate its rated capacity only a small fraction of the time. This results in “peaky power”, that is, most of the power is made over a relatively short period of time.
The high tip speeds of HAWTs mentioned above create another disadvantage for the large, conventional aerodynamic turbines. A 100 m swept area has a 314 m circumference and at 20 revolutions per minute (rpm), the tips travel 104 m/s. This is a fundamental limitation on the scalability of HAWT. The tip speed for larger swept areas is limited by the speed of sound and the specific strength of the blade material to withstand the centrifugal forces. This speed presents a fundamental risk for birds and from fatigue forces over time causing catastrophic blade failure. In the aerodynamic design, the blades are a relatively small percentage of the swept area, making it inviting for birds to try to fly through. The blade design is also the main reason that the aerodynamic design needs a relatively high wind speed just to start to spin. Combined with the friction from the gearbox and bearing systems, HAWTs are not effective in low wind speeds.
Wind speed is seldom constant and since the tip speed of a conventional HAWT is a multiple of the wind speed, there is significant variation in the speed of the rotor. This causes huge “on again-off again” loads that stress the longevity of gear boxes. Additionally, the speed change is on the wrong end of the gear box, which then increases the speed of the rotor 100 times. Consequently, a small change in the speed of the rotor will result in a large change in the speed at the generator. These factors combine to make the frequency of current generated highly variable and erratic. As a result, this requires expensive electricity to condition the grid. In most cases alternate current (AC) asynchronous generator current is rectified to direct current (DC). Then, the DC is inverted back to AC three-phase 60 Hz digitally (as a sine wave in little steps). There are capital costs, efficiency losses, cooling systems, power quality problems and maintenance issues that must be borne with this method.
When the focus of the industry changes from the MW rating of the turbine to useful load matching, there will be more interest in turbines optimized for average wind speeds. Vertical axis wind turbines (VAWTs) in an impulse configuration have a relatively high efficiency in lower wind speeds because of their higher blade areas and percentage of swept area. Although not as efficient, this design will make power most of the time the wind is blowing. This is more desirable for power companies and mitigates the need for time shifting or storing wind generated electricity.
The capital costs and the reliability of the gearboxes needed to step up the speed of the main shaft to a speed which is useful for generating electricity are other factors in wind power generation. The gearbox contains hundreds of precision parts. The quality of the bearings, the profiles of the gear teeth, the stiffness of the gearbox casing and many other issues make gearbox manufacturing a precision engineering art. Precision machine tools and skilled labor are required to construct the components for these gearboxes. Considering gearboxes account for approximately 30% of the cost of the new turbine, gearbox availability has been a limiting factor in the supply chain for wind turbines. Once in service, a failure of any single part is likely to result in the failure of the entire gearbox. The risk of new, larger machines and unproven gearbox designs will be an impediment to reaching offshore winds. Installation and maintenance costs of offshore turbines are three times the cost of land-based turbines, which has prevented the East Coast from having a single offshore wind turbine. The present invention would eliminate the costs associated with the gearbox and additionally result in shorter manufacture times for turbines.
The broad support base and low center of gravity in the VAWT conveniently enables flotation of the turbine. Trying to float a HAWT is comparatively much more difficult, because a mass fixed high on a heavy pole is fundamentally unstable. Flotation is a key design aspect of the present invention. Research by the NREL has confirmed the huge potential advantages of floating wind turbines, including estimates of over 1000 GW of estimated power in offshore wind resources surrounding the continental United States.
As mentioned above, the energy in the wind increases as the cube function of its velocity, so class 6 winds have more than double the energy of class 4 winds. Also the wind velocity near the surface is much higher and this reduces the need to elevate the turbine into the air. These two factors, along with a multitude of other advantages, effectively counteract the relative inefficiency of the vertical axis wind turbine.
Power transmission is also a problem associated with making wind power a viable energy solution. As noted before, the best winds on the continental United States are class 3 and 4 winds in the Great Plains and Mountain states which are 1500 miles from major load centers. However, there are class 6 winds just 30 miles offshore. Over 75% of the electricity consumed is along the coasts and Great Lakes which are nearly the best wind resources available to the United States. Undersea cables are much less expensive to permit and do not require high tension towers. And clearly, the 30 to 50 mile offshore range is significantly shorter than the 1500 mile run currently contemplated and needed to transmit power to and from the East Coast. Such shorter distances result in reduced costs and transmission losses. In fact, undersea cables have a very significant advantage in that they are insulated from summertime heat. Higher temperatures reduce the conductivity of transmission cables, so when the grid is most strained, during the summer heat, that heat reduces the transmission capacity. Alternatively, undersea cables are not subject to this loss, which is amplified by the longer length of landlines.
Power plants are often located near the coasts or Great Lakes for access to coal and cooling water. Many of these power plants have been decommissioned or are only used for peak load because of old turbine/generator technology. However, their connection to the grid still exists. They were intentionally located near the high demand centers and offer the ultimate in “smart grid recycling”, providing ready-made high amperage distribution points for offshore wind power.
An additional advantage of VAWTs over HAWTs involves wind direction and maximizing power generation. VAWTs are not sensitive to wind direction and do not require being pointed into the wind. In contrast, HAWTs need to be pointed into the wind, and so far, there has been no reasonable plan to deal with this issue except for huge, economically impractical floats. The proposed 3-point floatation provides convenient places for three mooring tethers to provide the required anti-rotation. The VAWT will always wind up and tighten its tethers in the same direction, while the HAWT needs to be actively pointed into the wind. Accordingly, there is a need to provide VAWT capable of generating power offshore.
The floating VAWT would address many of the technology and policy problems of a marine-based HAWT. Because the turbines solve the flotation problem, no foundation is required on the sea floor. This is a huge reduction in marine citing costs, making them cheaper to site than land-based turbines. The VAWT would be built on shore, towed out to a field of mooring anchors, tied up and plugged in. No crane or assembly would be required at sea, again resulting in an order of magnitude cost-reduction.
Recent policy decisions by high-ranking government officials indicate that offshore wind energy is becoming a top priority. The proposed invention for floating offshore wind turbines is not just consistent with, but enables, the new national policy direction by eliminating policy, cost and technical roadblocks as mentioned above.
Because the wind farms for the VAWTs would be located in deep water which have been off-limits to HAWT, there is not an either-or choice between the turbine technologies. VAWTs may be seen as an additional layer of wind energy capacity that can be built on top of the already existing wind turbine manufacturing industry. Due to the higher quality winds and lower costs associated with VAWTs, there is a need for this turbine technology.